Mine

Index: twisted/web2/http_headers.py
===================================================================
--- twisted/web2/http_headers.py        (revision 15014)
+++ twisted/web2/http_headers.py        (working copy)
@@ -281,6 +281,12 @@
 
 
 class MimeType:
+    def fromString(clazz, mimeString, params={}, **kwargs):
+        mediaType, mediaSubtype = mimeString.lower().split("/")
+        return clazz(mediaType, mediaSubtype, params, **kwargs)
+
+    fromString = classmethod(fromString)
+
     def __init__(self, mediaType, mediaSubtype, params={}, **kwargs):
         self.mediaType = mediaType
         self.mediaSubtype = mediaSubtype